At present, as a general cancer therapy, e.g., radiation therapy, chemotherapy, immunotherapy and surgical (excision) therapy are mentioned. The chemotherapy is a method for suppressing cancer by use of an anticancer therapeutic agent made of various types of low-molecular compounds.
The therapy using an anticancer therapeutic agent is directed to reduce the size of a solid tumor. However, the most part of a tumor is occupied by differentiated cancer cells which no longer have a function as a cancer stem cell and it is pointed out in a general anticancer agent treatment that the differentiated cancer cells are only targeted to reduce the size thereof.
Cancer has cells having nature of stem cells, called cancer stem cells. The cancer stem cells, which were first identified in 1997 in an acute myeloid leukemia, are now increasingly found in various types of cancers including solid cancers, and recently, a new way of thinking, called “cancer stem cell hypothesis” that cancer would be developed from cancer stem cells as an origin, has been proposed (NPL 1).
According to the hypothesis, even though the most part of cancer cells are killed or excised out by applying the aforementioned therapy, if a very small number of self-reproducible cancer stem cells remain, recurrence and metastasis conceivably occur. In short, it is considered that recurrence and metastasis are caused by the remaining small amount of cancer stem cells. Accordingly, if cancer stem cells can be targeted and completely eradicated, it is expected to develop a useful therapy for preventing metastasis and recurrence of cancer.
It is pointed out that some of the cancer stem cells acquire drug resistance to an anticancer therapeutic agent (NPL 2).
At present, as a low-molecular compound for use in detection of cancer stem cells and as a therapeutic agent, a compound containing radioactive Cu-ATSM is known (PTL 1). However, the radioactive compound may affect normal cells. Therefore, when a radioactive compound is used, safety becomes a matter of concern. In addition, it is also pointed out that cancer stem cells may develop strong resistance to radiation.
In the circumstances, it has been desired to develop a drug inhibiting cancer stem cells and a compound capable of detecting cancer stem cells.